Heavy petroleum fractions, such as vacuum gas oil or resides may be catalytically cracked to lighter and more valuable products. The product of catalytic cracking is conventionally recovered and the products fractionated into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and diesel fuel; lube fractions; and heavier fractions.
Generally, sulfur occurs in petroleum and petroleum products as hydrogen sulfide, organic sulfides, organic disulfides, mercaptans, also known as thiols, and aromatic ring compounds such as thiophene, benzothiophene (BT), dibenzothiophene (DBT) and their alkylated homologs. The sulfur in aromatic sulfur-containing ring compounds will be herein referred to as xe2x80x9cthiophenic sulfurxe2x80x9d.
Where a petroleum fraction is being catalytically cracked and contains sulfur, the products of catalytic cracking usually contain sulfur impurities which normally require removal, usually by hydrotreating, in order to comply with the relevant product specifications. Such hydrotreating can be done either before or after catalytic cracking.
Conventionally, feeds with substantial amounts of sulfur, for example, those with more than 500 ppm sulfur, are hydrotreated with conventional hydrotreating catalysts under conventional conditions, thereby changing the form of most of the sulfur in the feed to hydrogen sulfide. The hydrogen sulfide is then removed by amine absorption, stripping or related techniques. Unfortunately, these techniques often leave some traces of sulfur in the feed, including thiophenic sulfur, which are the most difficult types to convert.
The ease of sulfur removal from petroleum and its products is dependent upon the type of sulfur-containing compound. Mercaptans are relatively easy to remove, whereas aromatic compounds such as thiophenes are more difficult to remove. Of the thiophenic sulfur compounds, the alkyl substituted dibenzothiophenes are particularly resistant to hydrodesulfurization.
Hydrotreating any of the sulfur containing fractions which boil in the distillate boiling range, such as diesel fuel, causes a reduction in the aromatic content thereof, and therefore an increase in the cetane number of diesel fuel. While hydrotreating reacts hydrogen with the sulfur containing molecules in order to convert the sulfur and remove such as hydrogen sulfide, as with any operation which reacts hydrogen with a petroleum fraction, the hydrogen does not only react with the sulfur as desired. Other contaminant molecules contain nitrogen, and these components undergo hydrodenitrogenation in a manner analogous to hydrodesulfurization. Unfortunately, some of the hydrogen may also cause hydrocracking as well as aromatic saturation, especially during more severe operating conditions of increased temperature and/or pressure. Typically, as the degree of desulfurization increases, the cetane number of the diesel fuel increases; however this increase is generally slight, usually from 1-3 numbers.
The current specification for diesel fuel permits a maximum sulfur content of 0.05 wt %. However, the EPA is expected to propose new diesel fuel specifications that will become effective in 2004. The new specification is likely to require further reduction of sulfur content in diesel fuels to below 50 ppmw. Recently, the European Union published new diesel specifications, which limit the sulfur content of diesel fuels to a maximum of 350 ppmw after the year 2000, and to 50 ppmw maximum after the year 2004. In addition, the specifications may require an increase in the cetane value of diesel fuels to 58 in the year 2005, and a reduction in the polyaromatics content.
Hydrotreating can be effective in reducing the level of sulfur to moderate levels, e.g. 500 ppm, without a severe degradation of the desired product. However, to achieve the levels of desulfurization that will be require by the new regulations, almost all sulfur compounds will need to be removed, even those that are difficult to remove such as DBTs. These refractory sulfur compounds can be removed by distillation, but with substantial economic penalty, i.e., downgrading a portion of automotive diesel oil to heavy fuel oil.
Thus, there remains a need for a method of removing sulfur from a hydrocarbon feed under moderate process conditions.
The present invention is a method for removing sulfur from an effluent produced by hydrotreating a hydrocarbon feed. A process is provided in which the sulfur remaining in the effluent from the hydrotreating process is removed by contacting the effluent with a noble metal containing hydrodearomatization (HDA) catalyst on a support under reaction conditions sufficient to hydrogenate at least one ring within the polyaromatic sulfur compounds. The hydrogenated DBTs are then desulfurized at a rate that is 10-50 times faster than the original aromatic parent molecules over the same noble metal catalyst or any other conventional hydrotreating catalyst.
In a preferred embodiment, the lighter fraction of effluent from the hydrotreating process is first separated from the heavier fraction of effluent before the heavier fraction is contacted with the hydrodearomatization catalyst. In another preferred embodiment, the H2S and NH3 produced by the hydrotreating step are removed before the effluent is contacted with said hydrodearomatization catalyst. In a further preferred embodiment, the H2S and NH3 are removed from the effluent, along with the lighter fraction, before the heavier fraction is contacted with the hydrodearomatization catalyst.
The support for the hydrodearomatization catalyst can be selected from the group consisting of gamma-Al2O3, zeolite beta, USY, ZSM-12, mordenite, TiO2, ZSM-48, MCM-41, SiO2, ZrO2, xcex7-Al2O3, ∂-Al2O3, SAPOs, MEAPOs, AlPO4s, or a combination thereof. The noble metal catalyst of the dearomatization catalyst can be selected from the group consisting of platinum, palladium, ruthenium, rhodium, iridium, osmium, rhenium, or a combination thereof. Platinum is preferred.
In another embodiment of the invention, the method further includes contacting at least the heavy fraction of the effluent from the hydrotreating process with a hydrodesulfuriztion (HDS) catalyst. It is preferred that the hydrodesulfurization catalyst contain a base metal. Typical HDS catalysts include, but are not limited to, CoMo/Al2O3, NiMo/Al2O3, NiW/Al2O3, and NiCoMo/Al2O3.
The effluent from the hydrotreating step is contacted with the hydrodearomatization catalyst and the hydrodesulfurization which are arranged within a reaction vessel. This arrangement within the reaction vessel can consist of various schematics. One schematic is with the hydrodearomatization catalyst and the hydrodesulfurization catalyst combined in two separate layers, or in multiple alternating layers. Another is with the hydrodearomatization catalyst and the hydrodesulfurization catalyst being separate extrudates which are mixed. Another schematic is with the hydrodearomatization catalyst and the hydrodesulfurization catalyst being a single extrudate in which the noble metal of the hydrodearomatization catalyst and the metal of the hydrodesulfurization catalyst are co-incorporated. The schematic can also be any combination thereof.
Process conditions will vary based upon the properties of the effluent feed. However, the preferred operating conditions generally include a temperature of 550-800xc2x0 F., a pressure of 200-1100 psig, an LHSV of 0.5-10 hrxe2x88x921, and a H2 recycle rate of 300-2500 SCFB.
Thus, the present invention provides a method of removing sulfur from a hydrocarbon feed at a lower temperature and pressure, and with lower capital investment.